Method and apparatus for testing dynamoelectric machine rotors

ABSTRACT

A method and apparatus for testing dynamoelectric machine rotors, particularly squirrel cage rotors for induction motors, to obtain resistance, reactance, and effective skew values to permit identification of rotor defects. The rotor is rotated in an alternating magnetic field and pick-up coils are used to sense the voltage generated in the rotor by sensing the magnetic flux generated by magnetization of the rotor during rotation. Current sensing is used to determine the current used in magnetizing the rotor and a separate skew pick-up coil is utilized to detect effective electrical skew. These signals are processed to determine whether the rotor meets predetermined pass/fail criteria, to provide detailed statistical data and to generate a failure indication responsive to one of the values falling outside respective predetermined limits.

This is a division, of application Ser. No. 860,240, filed May 6, 1986,now U.S. Pat. No. 4,801,877.

FIELD OF THE INVENTION

This invention relates generally to the field of testing rotors ofdynamoelectric machines, such as electric motors and generators, andmore particularly to a method and apparatus for testing squirrel cagerotors for induction motors to obtain the resistance, reactance, andeffective electrical skew of the rotor to permit identification of rotordefects.

BACKGROUND OF THE INVENTION

Squirrel cage rotors for modern induction motors typically include acore comprised of a stack of steel laminations and an aluminum squirrelcage conductor arrangement, usually formed as a die casting.Manufacturing techniques have been perfected to the point where theserotors are mass produced with a high probability of uniformity and highquality. There are, however, a number of possibilities for deficiencies,including porosity or impurities in the aluminum casting and opencircuits in the squirrel cage conducting bars which can affect theelectrical resistance of the rotor, poor insulation between the squirrelcage conductors and the iron core which can produce undesired variationsin the effective skew, and various other manufacturing defects. Thus itis desirable to test dynamoelectric machine rotors economically andreliably to detect such defects.

Because quality problems are generally infrequent, it is not economicalto perform expensive tests on every individual rotor. However, sincehidden defects do occur, in order to maintain a high degree of qualitycontrol there is a need to perform low cost tests on each rotor beforeit is assembled with a stator to form a complete machine. Further, itcan be desirable to obtain information on the resistance, reactance andeffective skew of the rotors for evaluation of defects, manufacturingprocesses and quality control.

A number of prior art methods have been developed in an attempt to testsquirrel cage rotors. Some, such as that disclosed in U.S. Pat. No.2,844,794, assigned to the assignee of the present invention, requirethe use of the dynamoelectric machine stator core, while others usedestructive testing techniques. One non-destructive prior art techniquefor testing rotors independent of the stator is disclosed in U.S. Pat.No. 3,861,025, assigned to the assignee of the present invention. Thistechnique involves rotating the rotor in a static magnetic field andevaluating the waveform of the resulting induced voltages displayed onan oscilloscope. This technique requires extensive operator training tointerpret the oscilloscope display, and has inherent limitations on theresults that can be achieved. Another prior art testing techniqueutilizes a stator fixture excited by a fixed AC current into which therotor is placed and manually rotated to obtain a peak power measurement(i.e. power into the rotor) using a pick-up coil. By using the currentmeasurement, the impedance of the rotor can be obtained, but separateresistance, reactance and skew information can not be determined.

It is accordingly an object of the present invention to provide a noveland improved method and apparatus for non-destructive testing ofdynamoelectric machine rotors.

It is another object of the invention to provide a novel, economical,and reliable method and apparatus for non-destructive measurement of theresistance and reactance of dynamoelectric machine rotors.

It is yet another object of the invention to provide a novel, economicaland reliable method and apparatus for non-destructive measurement of theeffective skew of dynamoelectric machine rotors.

It is yet another object of the invention to provide a novel,economical, and reliable method and apparatus for non-destructivetesting of dynamoelectric machine rotors which provides automaticpass/fail determinations.

It is still another object of the invention to provide a novel,economical, and reliable method and apparatus for non-destructivetesting of dynamoelectric machine rotors including the measurement ofresistance, reactance and skew and a detailed statistical comparison andevaluation of the measurement results, as well as automaticidentification of defective rotors.

SUMMARY OF THE INVENTION

Briefly, according to preferred embodiments of the invention, a testapparatus and method is provided for testing dynamoelectric machinerotors. The apparatus comprises a test head for accepting and causingrelative angular movement between the rotor and test head and includesexciting means for magnetizing the rotor during such angular movement inresponse to an alternating current. Voltage sensing means is providedfor generating a voltage signal responsive to the magnetic fluxvariations generated by the rotor in response to the magnetization bythe exciting means. Current sensing means is provided for sensing themagnitude of the alternating current utilized to magnetize the rotor andfor generating a current signal representative thereof. Processing meansis provided for determining the resistance and reactance of the rotorresponsive to the voltage signal and current signal.

In addition skew sensing means may be provided for sensing the effectiveelectrical skew of the rotor and for generating an effective skew signalresponsive thereto. The processing means is usable for determining aneffective electrical skew of the rotor responsive to the effective skewsignal.

The subject matter of the invention is particularly pointed out anddistinctly claimed in the claims at the concluding portion of thisspecification. The invention itself, both as to its organization andmethod of operation, together with further objects and advantagesthereof, may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic front view illustrating a specific embodimentof a dynamoelectric machine rotor test apparatus for testing squirrelcage rotors in accordance with the invention.

FIG. 2 is a cut-away perspective view illustrating a specific embodimentof a typical squirrel cage rotor.

FIG. 3 is a detailed block diagram illustrating a specific embodiment ofthe dynamoelectric machine rotor test apparatus for testing squirrelcage rotors in accordance with the invention.

FIG. 4 is a cut-away perspective view illustrating the coreconfiguration of a specific embodiment of the test head of the testapparatus illustrated in FIG. 3.

FIG. 5 is a cut-away diagrammatic view illustrating the structure of aspecific embodiment of the test head of test apparatus illustrated inFIG. 3.

FIG. 6 is a cross sectional view illustrating the skew winding portionof a specific embodiment of the test head of the test apparatusillustrated in FIG. 3.

FIG. 7 is a diagrammatic view illustrating the structure of a specificembodiment of the test head of the test apparatus illustrated in FIG. 3.

FIG. 8 is a diagrammatic view illustrating a laid open structure of aspecific embodiment of the test head of the test apparatus illustratedin FIG. 3.

FIG. 9 is a diagrammatic view illustrating a specific embodiment of thetest fixture structure of the test apparatus illustrated in FIG. 1 withthe test fixture in the rotor extended position.

FIG. 10 is a diagrammatic view illustrating a specific embodiment of thetest head and mechanical structure of the test apparatus illustrated inFIG. 1 with the test head in the rotor retracted position.

FIG. 11 is an expanded diagrammatic view illustrating a specificembodiment of the test head and rotor clutch mechanism illustrated inFIG. 9 in the rotor extended position.

FIG. 12 is an expanded view of a portion of the rotor clutch mechanismillustrated in FIG. 11.

FIG. 13 is an expanded diagrammatic view illustrating a specificembodiment of the test head and rotor clutch mechanism illustrated inFIG. 10 in the rotor retracted position.

FIG. 14 is an expanded view of a portion of the rotor clutch mechanismillustrated in FIG. 13.

FIG. 15A is a flow diagram illustrating the program flow for oneembodiment of the data processor of FIG. 3.

FIG. 15B is a flow diagram illustrating the program flow for oneembodiment of the control processor of FIG. 3B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a general diagrammatic front view illustrating a preferredembodiment of the dynamoelectric machine rotor test apparatus 20according to the invention. The test apparatus 20 is a dual test fixtureembodiment having two test fixtures 22, 24. Other embodiments utilizingone fixture or more than two fixtures will be apparent to those skilledin the art in view of the disclosure provided hereinafter. The two testfixtures 22, 24 each respectively comprise a test head 26, 28 and ahydraulically driven retraction and drive mechanism 36, 38. Theretraction and drive mechanism 36, 38 functions to retract and rotatethe rotor within the test head in response to activation of a startbutton 32, 34 associated with the respective test fixtures 22, 24. Thetest fixtures 22, 24 are mounted, as shown, in a test stand 30 toprovide convenient access by an operator.

Each test fixture 22, 24 is coupled to a data acquisition, processingand control system 40 mounted in a rack 42 as shown. The system 40comprises data acquisition and processing circuitry in a drawer 51,coupled to a terminal 44, and to a printer (e.g., an Epson RX-80)contained in a drawer 53, and coupled to a power supply 54. The terminal44 comprises a display 46 (e.g., a Computerwise, Inc., Transterm ModelTM-71 LCD Display) for displaying test results, control information, andother data, and a keyboard 48, (e.g., a Computerwise, Inc. TranstermModel TM-71 16-key, alpha/numeric keyboard) for entry of data andcontrol information. The printer permits the printing of results andother data, while the power supply 54 provides electrical power for allof the electrical elements of the apparatus 20.

The single common data acquisition, processing and control system 40controls testing and acquires data from each test head 22, 24independently. In response to initiation of a test on a respectivefixture 22, 24 by a operator, the system 40 automatically performs therotor test on the respective test fixture. Thus, once the test sequenceis initiated by the operator, the system 40 controls the rotation of therotor within the fixture, the acquisition of data via the fixture, andthe processing of the acquired data without further operatorintervention.

A typical squirrel cage rotor suitable for testing by the apparatus 20is illustrated in FIG. 2. The rotor 60 includes a cylindrical core 62formed of a stack of laminations made of a magnetic material such asiron. The rotor core 62 includes a center opening 65 which runs axiallythrough the center of the rotor 60 and which is intended to be mountedon the rotor shaft (not shown). The rotor core 62 also includes acircumferential series of nearly axial slots 64 near the outer diameterof the rotor 60. These slots may be disposed in a skewed or inclinedrelationship with respect to the longitudinal axis of the rotor. Thesquirrel cage windings are provided by an aluminum casting 66 disposedin and about the rotor core 62 comprising conductive bars 67 which fillthe slots 64 and conductive end rings 68, 69 integral with theconductive bars.

This structure will have inherent resistance and reactancecharacteristics which are highly dependent on the proper construction ofthe rotor such as proper formation of the conductive bars in the slots64. In addition, the skew characteristics of the rotor are largelydetermined by the angle of incline (i.e. skew) of the conductive barsoff of the true longitudinal axis. However, variations of the propertiesof the magnetic material, the aluminum casting, the iron to aluminuminsulation, etc. will produce variations in the effective electricalskew (i.e., the skew as measured by its effects on the electromagneticfield in the air gap).

The test heads 26, 28 of the test fixtures 22, 24 have a uniqueconstruction which may best be understood by reference to FIGS. 4-8. Thetest heads 26, 28 comprise a structure utilizing a core of magneticmaterial 200 very similar to a conventional dynamoelectric machinestator as illustrated in FIG. 4. This core is formed in the conventionalmanner of a stack of laminations of magnetic material such as iron,shaped to provide a plurality of slots which permit a set of windings tobe arranged in the slots as illustrated in FIG. 5.

FIG. 5 is a diagrammatic illustration of the structure of a test head 26(also see FIG. 7). The test head includes a set of primary windings 210which form at least one pair of poles 212, 214 as illustrated in FIG. 7.These primary windings form the exciting current carrying winding forthe test head 26 which, when an alternating current is supplied during arotor test, creates an alternating magnetic field in a centercylindrical cavity 220. For testing, the rotor is positioned within thecenter cavity 220 and rotated, thereby inducing voltages in the rotor.This results in induced currents in the rotor and consequentlygeneration of magnetic flux by the rotor which is sensed by the pick-upcoil 106. The pick-up coil 106 comprise a set of coil windings in whichis generated a voltage representative of the voltage induced in therotor. These coils are, in the illustrated embodiment, composed of amultiple turn loop (any number of turns may be used), as shown, coupledin series to provide the voltage signal. In the preferred embodiment,these voltage pick-up coils 106 are wound over the primary coils 102.

The test head 26 also includes a skew pick-up coil 110 located at oneend of the test head structure 26. This skew pick-up coil 110 ispositioned in quadrature with the poles 212, 214 and at the end of thecore 200 to sense flux build-up at the end of the rotor due to the skewcharacteristics of the rotor. In the illustrated embodiment, the skewpick-up is composed of two multiple turn loops coupled in series, asshown, although other coupling configurations and any number of turns(N) may be used. The skew pick-up coils 110, in the illustratedembodiment, are positioned within a groove 222 near the end of the core200, as may best be understood by reference to FIG. 6.

For a further understanding of the structure of the coils of the testhead 26, reference may be made to FIG. 8 which shows a diagrammatic viewof the test head 26 laid flat. The primary windings 102 are shownforming two poles 212, 214 with the voltage pick-up coils 106 wound insome of the slots among the primary coil windings 102. The skew pick-upcoil 110 is shown in quadrature relationship to the primary windings atone end of the core 200.

Referring now to FIG. 3, there is shown a detailed block diagramillustrating a specific embodiment of the dynamoelectric machine rotortest apparatus 20. Each test head 26, 28 includes an excitation means102, 104 composed of the set of current carrying windings which producean alternating magnetic field when energized by an alternating currentof predetermined magnitude (e.g., 60 hz at 2.4 amps in the illustratedembodiment) coupled from the power supply 54, as shown. The magneticfield produced will magnetize a rotor rotated within the field producingmagnetic flux which is dependent upon the rotor characteristics.

Each head 26, 28 also includes the voltage sensing pick-up coil 106, 108responsive to the rotor induced magnetic flux which produces a voltagesignal representative of the voltage induced in the rotor. The skewsensing pick-up coil 110, 112 is also located in the test head 26, 28which produces an effective skew signal responsive to flux build-up atthe end of the rotor due to the rotor's effective skew. Each of thesesense signals is coupled to a sample and hold circuit 120, as shown(e.g., a Burr-Brown ADSHC-85). A current sensor 114 (e.g., aconventional current transformer), coupled as shown to the supply 54,senses the current provided to energize the test heads 26, 28 andcouples a current sense signal to the sample and hold circuit 120.

Also coupled to the power supply 54 is a phase lock loop 122 (e.g., aNational CD4046) which generates timing pulses which are phase locked tothe exciting alternating current supplied to the test head windings 102,104. In the illustrated embodiment, there are 32 pulses generated foreach cycle of the exciting alternating current such that each pulse isgenerated at the same phase of the cycle for each succeeding cycle.These phase locked timing pulses are coupled, as shown, to the sampleand hold circuit 120 to synchronize the sampling of the sense signalscoupled from the voltage pick-up 106, the skew pick-up 110, and thecurrent sensor 114. The phase locked timing signals are also coupled toa data processor 140 via a conductor 127, as shown.

The sample and hold circuit 120 and the phase locked loop circuit 122are part of an analog to digital system 130 which also includes ananalog multiplexor 124 (e.g., an Analog Devices AD7506) and an analog todigital converter 126 (e.g., Analog Devices ADC1131 high speed, 14 bitconverter) configured as shown. The analog to digital system 130 is asubsystem of the data acquisition and processing circuit 50. The dataacquisition and processing circuit 50 controls the acquisition of thetest data and processes the data to produce useful test results as wellas rotor pass/fail determinations. The data acquisition and processingcircuit 50 also includes the data processor to 140 (e.g., an Intel 86/14microcomputer) and a control processor 150 (e.g., an Intel 86/35microcomputer) as shown. This multi-computer system provides highlyefficient data acquisition and processing, although other configurations(e.g., a single microcomputer system) may also be used.

During a rotor test, the sample and hold circuit 120 simultaneouslysamples each of the sense signals each time a timing pulse from thephase locked loop 122 occurs. Simultaneous sampling of current andvoltage sense signals permits calculation of a power value (W) (note:simultaneous sampling of the skew signal is not needed to permit thecalculation of a power value). These samples, taken by the sample andhold circuit 120 are coupled to an analog multiplexer circuit 124, asshown. The analog multiplexer 124 multiplexes the samples sequentially,under the control of the data processor 140, to an analog to digitalconverter 126. The analog to digital converter digitizes the samples andcouples the digitized samples to the data processor 140. The digitizedsamples coupled to the data processor 140 are processed to reduce thedata to usable form.

In the illustrated embodiment, the processor 140 acquires 32 samples ina cycle of the exciting alternating current (i.e., at 60 hz, one sampleevery 520 microseconds), then ignores samples for the next five cycles,and samples again for 32 samples. (The flow of program control for theprocessor 140 may be more fully understood by reference to the flowchart 260 illustrated in FIG. 15A in conjunction with the followingdescription). This pattern is continued for a total of forty samplingcycles of 32 samples each to complete one rotor test sampling sequencein four seconds. Once the data is acquired for each sample cycle, theprocessor 140 multiplies each current sample by the correspondingvoltage sample to obtain a power value W (where W is power into therotor). The 32 samples of the voltage signal, current signal, skewsignal, and power value are then processed to obtain four test valueswhich are a mean power value (W), and a root means square (rms) valuefor the voltage (V), current (I), and skew (SK) signals. This process isrepeated 40 times, once for each sample cycle, thereby obtaining 40 setsof the four test values.

These forty sets of test values are coupled from the data processor 140to the control processor 150 at the end of a rotor test sequence. (Theflow of program control for the control processor 150 may be more fullyunderstood by reference to the flow chart 270 illustrated in FIG. 15B inconjunction with the following description). The control processor 150then determines a mean resistance (R), reactance (X), and effective skew(ESK) for the rotor from the 40 test values, as well as the range of the40 values for the resistance (referred to as dissymmetry, DS) and theeffective skew (referred to as skew dissymmetry, DSK).

Each resistance value (R) is determined by the formula

    R=W/I.sup.2.

Each reactance value is determined by the formula

    X=((VI).sup.2 -W.sup.2)1/2)/I.sup.2.

The effective skew is determined by the formula

    ESK=SK/(I.sup.2 ×N)

where N=the number of turns of the skew pick-up coil.

Once the resistance, reactance, and effective skew values have beendetermined, the data is scanned to determine the maximum and minimumresistance and effective skew values. In addition, an average value ofresistance, reactance, and effective skew is determined by summing theforty values for each and dividing by forty. These values are stored ininternal memory within the data processor 140.

After all of the values have been calculated, the average value forresistance, reactance, and effective skew are each compared topredetermined maximum and minimum threshold values. In addition, thedissymmetry value is compared to a predetermined maximum. The maximumand minimum values may be entered through the key board 48 by theoperator prior to the beginning of a test run. If the calculated valuesfor the rotor are within the predetermined maximum and minimum thresholdvalue, then the rotor is passed as a good rotor. However, if the rotorhas any value outside of the predetermined limits, a fail (reject)indication is provided to the operator by means of an indicator such asa light or audible signal (not shown) to indicate a defective rotor. Thereject signals to activate the fail indicators are generated on outputs162 and 164, as shown.

In addition to the calculated values, additional statistical informationis also determined and stored on a Winchester magnetic disk 55 coupledto the control processor 150, as shown. Among the types of data storedon the Winchester disk 55 are totals of the number of passed rotors, thenumber of fail rotors including how many failed for each threshold,running sums of each of the calculated values, and running sums of thesquares of each of the values. This data permits the determination ofstatistical information over numerous tests of a test run, includingsuch information as averages and standard deviation. All the calculatedvalues of resistance, reactance, effective skew, dissymmetry, and skewdissymmetry for each test are displayed on the display 46 at the end ofa test. In addition, the printer 52 may be used to print the results ofa test as well as the statistical data. The printer 52 is activated bythe operator via commands from the keyboard 48.

The control processor 150 also controls the sequence of events thatoccur during a test. Various input and output signals are coupledbetween the control processor 150 and an opto-isolator 160 (e.g., anopto-22) via a bus 166, as shown. The opto-isolator provides a controlinterface to the test fixtures 22, 24. The start switches 32, 34 arecoupled to the opto-isolator 160 which couples the start signal to theprocessor in response to activation of one (i.e., Right (R) or Left (L))of the start switches 32, 34. In response, the control processor 150couples a control signal through the opto-isolator 160 to theappropriate retraction and drive mechanism 36, 38 (described in greaterdetail hereinafter with reference to FIGS. 9-14) which activates themechanism 36, 38 thereby starting the rotor test. The retraction anddrive mechanism 36, 38, in response to activation, retracts a rotorplaced on a test head 26, 28. When the rotor is fully retracted suchthat it is in place for testing, a position sensor 170, 172 (e.g., aconventional limit switch) generates a position signal which is coupledthrough the opto-isolator 160 to the control processor 150 viaconductors 174, 176. In response to the position signal, the controlprocessor 150 generates a drive signal which is coupled through theopto-isolator 160 to the drive motors 180, 182 via the conductors 184,186. This drive signal activates the motor 180, 182 to rotate the rotor.

In the illustrated embodiment, the rotor is rotated at a rate of 1revolution in four seconds, and is rotated one full rotation for acomplete test sequence (i.e., rotation for four seconds). During thefour second test sequence, the data acquisition and processing system 50acquires the desired data after which three seconds are utilized for thedata to be processed. The use of the two fixture system permits theoperator to set up a rotor on the unused fixture during this sevensecond test sequence. Thus, the dual fixture system allows moreefficient testing by reducing delays due to the operator set up time. Italso increases the cost effectiveness of the apparatus because bothfixtures can be controlled with a single processing system.

During operation, a test is initiated by an operator by placing a rotorto be tested onto the test head, for example, head 26. The operator theninitiates the test sequence by activating the start button 32, whichsignals the control processor 150 to activate the retraction mechanismthereby retracting the rotor to the test position. Once fully retractedthe position sensor 170 generates a signal coupled to the controlprocessor 150 which causes the control processor 150 to generate themotor activation signal, which activates the motor 180 to rotate therotor. The rotor is rotated at a rate of one rotation in four seconds,and one complete test sequence is completed in one rotation. During thefour second rotation period the voltage sensor 106, skew sensor 110, andcurrent sensor 114 are sampled by the sample and hold circuit 120.

The sample and hold circuit 120 is timed synchronously with the excitingalternating current applied to the coils 102 by timing signals from thephase lock loop circuit 122. During this test sequence, 32 samples aretaken during one cycle of the alternating current exciting signal, andone set of samples are taken every sixth cycle producing a total offorty sets of data. This data is coupled to the data processor 140 whichdoes the initial processing of the data and couples the results to thecontrol processor 150. The control processor 150 then performs the finalprocessing, calculating resistance, reactance, skew, dissymmetry andskew dissymmetry. These values are displayed on the display 46 and maybe printed on the printer 52 in response to commands entered through thekeyboard 58. Information to permit statistical analysis over a series oftests is then stored on a Winchester disk 55.

Referring now to FIG. 9, there is shown a detailed diagrammatic viewillustrating a specific embodiment of the test fixture structure 22 onwhich a rotor 60 has been placed in the extended position. Duringoperation, the rotor 60 is retracted to the test position as illustratedin FIG. 10. The test fixture 22 comprises the test head 26 and theretraction and drive mechanism 36. Located coaxially at the center ofthe center cavity 220 of the test head 26 is a spindle 230 over whichthe rotor 60 may be placed, as shown.

The spindle 230 comprises a shaft 234 having an upper cylindrical cap232 with a greater diameter than the shaft 234, and an annular ring 236at the lower end through which the shaft 230 is slidably positioned, asshown. The annular ring 236 is mounted on a cylindrical mount 238 whichis coupled by a spring loaded coupling to a shaft 240. The shaft 240 isslidably mounted in an aperture in the test stand 30 as shown. The shaft230 is threadedly coupled to the shaft 240 and the shaft 240 is coupledto a drive motor 280 which rotates the rotor 60 when the motor isactivated. This shaft-motor assembly is mounted on a bracket 242 whichslidably engages a slide shaft 244. The bracket 242 is connected to ashaft 246 of a hydraulic cylinder 250 which is powered by an externalsource (not shown).

In the extended position, the rotor 60 extends above the test head 26when the entire retraction and drive mechanism 36 is in its upper-mostposition as shown in FIG. 9. When activated, the hydraulic cylinder 250retracts the shaft 246 lowering the retraction and drive mechanism 36 tothe position shown in FIG. 10. This lowers the rotor to the retractedposition within the central cavity 220 of the test head 26. The rotor isthen rotated by the drive motor 180 which is activated when a positionsensor (see FIG. 3) detects that the mechanism 36 is in the retractedposition.

The rotor 60 is tightly held in position during rotation by a clutchmechanism more readily understood by reference to FIGS. 11-14. FIG. 11is an expanded view of the top portion of the test fixture 22 in theextended position. The spindle 230, as illustrated in FIG. 11, comprisesa set of annular sleeves 252 slidably positioned around a shaft 234, asshown. Between each sleeve 252 is an o-ring 254. These elements are heldin place by the annular ring 236 and the cap 232. In the extendedposition, the o-rings are not compressed, and therefore do not extend inthe radial direction beyond the edges of the annular sleeves 252, asillustrated in FIG. 12. Thus, the rotor 60 can easily slide over thespindle 230. However, when in the retracted position, as illustrated inFIG. 13, the annular ring 236 is pushed up against the annular sleeves252 due to the movement of the shaft 234 downward. This compresses theo-rings 254 causing them to extend radially beyond the edge of theannular sleeves 252 contacting the inner surface of the rotor centercavity as illustrated by FIG. 14. As a result, the rotor 60 is securelyheld in place by the frictional force exerted on the rotor 60 by theextended o-rings 254. Thus, the rotor can be easily mounted on thespindle 230 when in the extended position but the rotor is securely heldwhen in the retracted position.

Preferred embodiments of the novel method and apparatus for testingdynamoelectric machine rotors have been described for the purpose ofillustrating the manner in which the invention may be made and used. Itshould be understood, however, that implementation of other variationsand modifications of the invention in its various aspects will beapparent to those skilled in the art, and that the invention is notlimited by the specific embodiments described. It is thereforecontemplated to cover any and all modifications, variations orequivalents that fall within the true spirit and scope of the basicunderlying principles disclosed and claimed herein.

What is claimed is:
 1. A method for testing dynamoelectric machine rotors, comprising the steps of:imparting angular movement to a rotor, and magnetizing the rotor utilizing alternating current during such angular movement; generating a voltage signal in response to magnetic flux generated by the rotor in response to the magnetization of the rotor during rotation; sensing the magnitude of the alternating current utilized to magnetize the rotor and generating a current signal responsive thereto; and determining at least one of the resistance and reactance of the rotor based upon the voltage signal and current signal.
 2. The method of claim 1 further comprising the steps of sensing the effective electrical skew of the rotor and generating an effective skew signal responsive thereto, and wherein the step of determining further comprises calculating an effective electrical skew value in response to the effective skew signal.
 3. The method of claim 2 wherein the step of determining further comprises the step of simultaneously sampling the current signal and voltage signal in synchronization with the alternating current to produce sampled signals.
 4. The method of claim 3 wherein the step of simultaneously sampling further comprises generating synchronization pulses to synchronize the sampling with the alternating current.
 5. The method of claim 3 wherein the step of simultaneously sampling further comprises converting the sampled signals to digital data.
 6. The method of claim 5 further comprising the step of processing the digital data to determine the resistance and reactance of the rotor.
 7. The method of claim 6 further comprising the step of processing the digital data to determine an effective skew value of the rotor.
 8. The method of claim 7 further comprising the step of processing the digital data to determine dissymmetry and skew dissymmetry values of the rotor.
 9. The method of claim 8 further comprising the steps of comparing at least one of the resistance, reactance, effective skew, dissymmetry and skew dissymmetry values to respective predetermined limits and generating a failure indication responsive to one of the values falling outside its respective predetermined limits.
 10. The method of claim 1, wherein the step of imparting angular movement comprises the steps of creating a slip-free contact with the rotor, retracting the rotor into a test head and rotating the rotor within the test head for a predetermined amount of rotations at a predetermined rate in response to activation of a start switch.
 11. The method of claim 10 wherein the step of creating a slip-free contact comprises compressing a compressible o-ring mounted on a spindle adapted for axially receiving the rotor to cause the o-ring to expand radially against the rotor to thereby form a gripping contact.
 12. The method of claim 2 wherein the step of generating an effective skew signal further comprises the step of sampling the effective skew signal in synchronization with the alternating current to produce sampled effective skew signals.
 13. The method of claim 12 wherein the step of determining further comprises the step of converting the sampled effective skew signals to digital data and processing the digital data to calculate an effective skew value of the rotor.
 14. A method for testing dynamoelectric machine rotors, comprising the steps of:imparting angular movement to a rotor, and magnetizing the rotor utilizing an alternating current during said angular movement; sensing at least one characteristic related to at least one selected electrical characteristic of the rotor and generating sense signals in response thereto; and, calculating a value of at least one selected electrical characteristic of the rotor automatically in response to the sense signals.
 15. The method of claim 14 further comprising the steps of digitizing the sense signals to provide digital sense signals and processing the digital sense signals to determine a calculated value for at least one selected electrical characteristic of the rotor.
 16. The method of claim 14 wherein the step of sensing comprises sensing a plurality of different selected electrical characteristics and generating a plurality of sense signals and wherein the step of calculating comprises calculating values of a plurality of different electrical characteristics of the rotor in response to the plurality of sense signals. 